Revisiting five decades of 234Th data: a comprehensive global oceanic compilation

Abstract. We present here a global oceanic compilation of 234Th measurements that collects results from researchers and laboratories over a period exceeding 50 years. The origin of the 234Th sampling in the ocean goes back to 1967, when Bhat et al. (1969) initially studied 234Th distribution relative to its parent 238U in the Indian Ocean. However, it was the seminal work of Buesseler et al. (1992) – in which it was proposed that particulate organic carbon (POC) flux could be calculated from 234Th distributions if the ratio of POC to 234Th measured on sinking particles (POC : 234Th) at the desired depth was known – that drove the extensive use of the 234Th-238U radioactive pair to evaluate the efficiency with which photosynthetically fixed carbon is exported from surface ocean by means of the biological pump. Since then, a large number of 234Th depth profiles have been collected using a variety of sampling instruments and strategies that have changed the past 50 years. The present compilation is made of a total 223 datasets: 214 from studies published either in articles in referred journals, PhD thesis or repositories, and 9 unpublished datasets. The data were compiled from over 5000 locations spanning all the oceans for total 234Th profiles, dissolved and particulate 234Th concentrations, and POC : 234Th ratios (both sediment traps and filtration methods that include two sizes classes; 1–53 µm and < 53 µm). A total of 379 oceanographic expeditions and more than 56000 234Th and 18000 238U data points have been gathered in a single open-access, long-term and dynamic repository. This paper introduces the dataset along with informative and descriptive graphics. Appropriate metadata have been included, including geographic location, date, and sample depth, among others. When available, we also include water temperature, salinity, 238U data and particulate organic nitrogen data. Data sources and methods information (including 238U and 234Th) are also detailed along with valuable information for future data analysis such as bloom stage and steady/non-steady state conditions at the sampling moment. The data are archived on PANGAEA repository, with the dataset’s DOI doi.pangaea.de/10.1594/PANGAEA.918125 (Ceballos-Romero et al., 2021). This provides a valuable resource to better understand and quantify how the contemporary oceanic carbon uptake functions and how it will change in future.